This invention relates to a fuel injector for use in supplying fuel, under pressure, to a combustion space of a compression ignition internal combustion engine. The invention also relates to a method of assembling a fuel injector.
In order to reduce emissions levels, it is known to provide fuel injectors in which the total area of the openings through which fuel is delivered can be varied, in use. One technique for achieving this is to use two valve needles, one of which is slidable within a bore provided in the other of the needles to control the supply of fuel to some of the outlet openings independently of the supply of fuel to others of the outlet openings. However, such arrangements have the disadvantage that fuel may be able to flow between the inner and outer needles giving rise to substantially continuous delivery of fuel at a low rate.
European patent application EP 99304430.4 describes a dual valve needle fuel injector which overcomes this problem. The outer valve needle is provided with a bore within which a sleeve is located to retain an inner valve within the bore. The inner surface of the sleeve and the outer surface of the inner valve needle together define a clearance passage for fuel which enables sliding movement of the inner valve needle. The dimensions of the clearance passage determine the rate at which fuel is supplied to and from a fuel chamber upstream of the inner valve needle, defined by the bore and an upper end surface of the inner valve needle.
Fuel flow through the clearance passage exerts pressure on the sleeve and, thus, the sleeve can adopt a non-circular shape. This can change the fuel flow characteristics of the injector and may cause the inner valve needle to become stuck. However, if the clearance passage is too large and fuel leaves the fuel chamber too quickly, a cavity can form in the chamber which adversely affects the performance of the fuel injector. Thus, it is difficult to manufacture a fuel injector of this type without compromising the fuel injector performance in some way. Furthermore, it is difficult to manufacture the sleeve component of the fuel injector as its dimensions are small.
It is an object of the present invention to alleviate one or more of the aforementioned problems associated with the prior art.
According to a first aspect of the present invention there is provided a fuel injector comprising a nozzle body having a first bore defining first and second seatings, an outer valve member, slidable within the first bore and engageable with the first seating to control fuel flow from a first outlet opening located downstream of the first seating, the outer valve member being provided with a through bore, an inner valve member, slidable within the through bore and engageable with the second seating to control fuel flow from a second outlet opening, the through bore defining a step engageable with an enlarged part of the inner valve member, the through bore permitting, during fuel injector assembly, insertion of the inner valve member into the through bore through an end of the through bore remote from the first and second outlet openings.
The fuel injector in accordance with the present invention can be manufactured conveniently. As the through bore extends along the complete length of the outer valve member, the inner valve member can be inserted into the through bore through the upper end thereof. In known fuel injectors, the bore in the outer valve member only extends along a part of the length of the outer valve member. It is therefore necessary to assemble the fuel injector by inserting the inner valve member into the bore through the lower end of the valve member.
Preferably, the outer valve member includes a radially extending enlarged region which cooperates with a part of the first bore to guide axial movement of the outer valve member within the bore such that the outer valve member remains substantially concentric within the bore throughout axial movement.
In one embodiment of the invention, the fuel injector includes an upper, sealing member which is received in the upper end of the through bore, the sealing member being in sealing engagement with the through bore to prevent fuel discharge from the through bore through the upper end thereof at undesirable stages in the operating cycle.
In an alternative embodiment, the fuel injector includes vent means for permitting fuel upstream of the inner valve member to vent from the through bore.
Conveniently, the vent means may include an upper valve member slidably mounted within the upper end of the through bore, the through bore defining a valve seat for the valve member, the valve member being exposed to fuel pressure within a chamber defined within the through bore between the inner valve member and the upper valve member, whereby movement of the upper valve member away from the valve seat due to fuel pressure within the chamber permits fuel to vent from the chamber.
This provides the advantage that high pressure fuel does not become trapped within the chamber during fuel injector operation. Trapping of fuel within the chamber may otherwise degrade fuel quality and, hence, fuel injector performance. Conveniently, the upper valve member may be provided with flats or slots to increase the flow area for fuel venting from the chamber.
The upper valve member may be provided with resilient bias means, for example a compression spring, to bias the upper valve member into the valve seat. In this way, when the fuel injector is not being supplied with fuel, a substantially fluid tight seal is formed between the chamber and the upper valve member to prevent any residual fuel venting from the chamber.
The outer valve member may be provided with a guide member, coaxial with the outer valve needle, which serves to guide axial movement of the outer valve member within the first bore. The guide member may be integrally formed with the outer valve member.
The upper valve member may be spaced apart from the chamber by a spacer member such that the valve member is located towards an upper end of the through bore. This simplifies manufacture of the fuel injector.
According to a second aspect of the present invention, there is provided a method of assembling a fuel injector including the steps of;
providing a nozzle body having a first bore formed therein,
providing an outer valve member having a through bore formed therein, the through bore defining a step engageable with an enlarged part of an inner valve member to be received within the through bore,
inserting the inner valve member in the upper end of the through bore,
inserting an upper member in the upper end of the through bore, and
inserting the outer valve member, having the inner valve member and the upper member inserted therein, into the upper end of the first bore.
The upper member may be an upper valve member which permits fuel upstream of the inner valve member to vent from the through bore.
Fuel injector assembly can therefore be achieved conveniently by assembling each of the component parts separately prior to assembly of the fuel injector.